Making changes in response to LayoutUpdated will require you to wait for an additional LayoutUpdated event to fire, as your changed require the Layout to be updated again. Also, certain configurations of the chart in the current version cause it to not be fully ready when the first LayoutUpdated is fired. This mostly relates to auto-extents on axes, so you can avoid having to wait through additional LayoutUpdates by setting the extent of the axis labels manually. Here is an example of capturing an image after turning a column red using the current version of the chart.

The xaml:

<Grid x:Name="LayoutRoot">
        <Grid.RowDefinitions>
            <RowDefinition />
            <RowDefinition Height="Auto" />
        </Grid.RowDefinitions>
        <Image x:Name="img" Width="500" Height="500" />

        <Button Grid.Row="1" Content="Click" x:Name="click" Click="click_Click" />
    </Grid>

And the code behind:

 public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
        }

        private XamDataChart _chart;

        private void click_Click(object sender, RoutedEventArgs e)
        {
            _count = 0;
            var data = new TestData();

            //var chart = new Button() { Content = "hi" };
            var chart = new XamDataChart();
            _chart = chart;
            chart.Width = 500;
            chart.Height = 500;
            chart.Axes.Add(new NumericYAxis());
            chart.Axes.Add(new CategoryXAxis() { ItemsSource = data, Label = "{Label}" });
            chart.Series.Add(new ColumnSeries()
            {
                ItemsSource = data,
                ValueMemberPath = "Value",
                YAxis = (NumericYAxis)chart.Axes[0],
                XAxis = (CategoryXAxis)chart.Axes[1]
            });

            chart.Measure(new Size(500, 500));
            chart.Arrange(new Rect(0, 0, 500, 500));
            chart.UpdateLayout();
            chart.LayoutUpdated += new EventHandler(chart_LayoutUpdated);
        }

        private int _count = 0;

        IEnumerable<DependencyObject> Visuals(DependencyObject t)
        {
            for (int i = 0; i < VisualTreeHelper.GetChildrenCount(t); i++)
            {
                var child = VisualTreeHelper.GetChild(t, i);
                yield return child;
                foreach (var subChild in Visuals(child))
                {
                    yield return subChild;
                }
            }
        }

        void chart_LayoutUpdated(object sender, EventArgs e)
        {
            switch (_count)
                {
                    case 0: 
                    //chart isn't ready
                    break;
                    case 1:
                    MakeFirstRed();
                    break;
                    case 2:
                    RenderTargetBitmap bmp = new RenderTargetBitmap(500, 500, 96, 96, PixelFormats.Pbgra32);
                    bmp.Render(_chart);
                    img.Source = bmp;
                    _chart.LayoutUpdated -= chart_LayoutUpdated;
                    break;
                }

                _count++;
        }

        private void MakeFirstRed()
        {
            var rect = Visuals(_chart.Series[0]).OfType<Rectangle>().Last();
            rect.Fill = new SolidColorBrush(Colors.Red);
        }
    }

    public class TestData
        : ObservableCollection<TestDataItem>
    {
        public TestData()
        {
            Add(new TestDataItem()
            {
                Label = "A",
                Value = 1
            });
            Add(new TestDataItem()
            {
                Label = "B",
                Value = 2
            });
            Add(new TestDataItem()
            {
                Label = "C",
                Value = 3
            });
            Add(new TestDataItem()
            {
                Label = "D",
                Value = 4
            });
        }
    }

    public class TestDataItem
    {
        public string Label { get; set; }
        public double Value { get; set; }
    }

Hope this helps!
-Graham